1. Field of the Invention
A method of manufacturing a hub unit for supporting a wheel and a die for manufacturing it of the present invention are utilized for manufacturing a hub unit for pivotally supporting a wheel of an automobile by a suspension.
2. Description of the Related Art
A wheel of an automobile is supported by a suspension via a hub unit for supporting the wheel. FIG. 10 is a view showing an example of the hub unit for supporting a wheel described in the U.S. Pat. No. 5,490,732. This hub unit 1 for supporting a wheel includes: a hub body 2, a pair of inner rings 3a, 3b, an outer ring 4, and a plurality of rolling elements 5, 5. In this arrangement, in the outside end portion of the outer circumferential face of the hub body 2, there is provided a flange 6 for supporting a wheel. In this case, the outside of the hub body is defined as a side which becomes an outside in the width direction when the hub is assembled to an automobile, that is, the outside of the hub body is the left in FIG. 10. On the contrary, the inside of the hub body is defined as a side which becomes a portion close to the center of an automobile, that is, the inside of the hub is the right in FIG. 10. At a base end portion of the flange 6 close to the center of the hub body 2, there is provided a step portion 7.
The pair of inner rings 3a, 3b, are externally engaged with a portion located in a range from the middle portion to the inside end portion of the hub body 2. An outside end face of the inner ring 3a, which is arranged outside, butts to a step face of the step portion 7, and an outside end face of the inner ring 3b, which is arranged inside, butts to an inner end face of the inner ring 3a which is arranged outside. In this connection, when a view is taken from the inner ring 3b which is located inside, the inside end face of the inner ring 3a, which is located outside, becomes a step face of the step portion. In the inside end portion of the hub body 2, there is provided a cylindrical portion 8. A forward half portion of this cylindrical portion 8, which protrudes inside from the inside end face of the inner ring 3b, is plastically deformed in the radial outward direction so that the calking portion 9 can be formed. The pair of inner rings 3a, 3b are interposed between the calking portion 9 and the step face of the step portion 7.
Between the pair of outer ring raceways 10, 10 (the first and the second raceways) provided on the inner circumferential face of the outer ring 4 and the inner ring raceways 11, 11 (the first and the second inner ring raceways) provided on the outer circumferential faces of the inner rings 3a, 3b, a plurality of rolling elements 5, 5 are respectively arranged. In this connection, in the example shown in the drawing, balls are used as the rolling elements 5, 5, however, in the case of a hub unit for supporting a wheel used for an automobile, the weight of which is heavy, tapered rollers may be used for the rolling elements. In some cases, the inner ring raceway 11 (the first inner ring raceway) arranged close to the flange 6 may be directly formed on an outer circumferential face of the hub body 2 so as to omit the inner ring 3a which is located outside. In this case, the step portion 7 is formed at a position corresponding to the inside of the outside inner ring 3a shown in FIG. 10.
In order to assemble the hub unit 1 for supporting a wheel into an automobile, the outer ring 4 is fixed to the suspension by the attaching portion 12, the shape of which is formed into an outward flange-shape, which is formed on an outer circumferential face of the outer ring 4, and a wheel is fixed to the flange 6. As a result, the wheel can be pivotally supported by the suspension.
Further, Japanese Unexamined Patent Publication No. 10-272903 discloses a structure in which damage such as cracks seldom occurs in the calking portion when the inner ring is fixed and the inner diameter of the inner ring and the diameter of the inner ring raceway formed on the outer circumferential face of the inner ring are difficult to be changed when calking work is carried out. FIGS. 11 to 15 are views showing the second example of the conventional structure disclosed in the above patent publication and also showing a conventional manufacturing method.
As shown in FIG. 14, before the cylindrical portion 8a is calked and expanded in the radial outward direction, the wall thickness of the cylindrical portion 8a, which is formed in the inside end portion of the hub body 2 so that the calking portion 9a for fixing the inner ring 3 can be composed, is gradually reduced when it comes to the forward end edge. Therefore, on the inside end face of the hub body 2, there is provided a tapered hole 13, the inner diameter of which is gradually reduced when it comes to the inner portion.
In order to calk and expand the forward end portion of the cylindrical portion 8a so that the inner ring 3 can be fixed to the inside end portion of the hub body 2, the hub body 2 is fixed so that it can not be moved in the axial direction, and the holding piece 14 holds the outer circumferential face of the inner ring 3 as shown in FIG. 15. While the hub body 2, on the outer circumferential face of which the inner ring 3 is engaged, is being prevented from fluctuating in the radial direction, as shown in the drawing, the die 15 is strongly pressed against the forward end portion of the cylindrical portion 8a. A central portion of the forward end face (lower end face shown in FIG. 15) of the die 15 is formed into a protruding portion 16, the shape of which is a circular truncated cone, which can be freely pushed into the cylindrical portion 8a. In the periphery of this protruding portion 16, there is provided a recess portion 17, the section of which is an arc, which surrounds the overall circumference of the protruding portion 16.
On the other hand, in the periphery of the inside end opening of the inner ring 3 to be fixed to the inner end portion (the right end portion shown in FIG. 11, the upper end portions shown in FIGS. 12 to 15) of the hub body 2 by the calking portion 9a, there is provided an inside end face 18 which is a flat face perpendicular to the central axis of this inner ring 3. The inner circumferential edge of the inside end face 18 and the inner circumferential face 19 of the inner ring 3, which is a cylindrical face, are continued to each other by the chamfer portion 20.
The calking portion 9a for pressing the inner ring 3, the shape of the inside end portion of which is formed as described above, against the step portion 7 of the hub body 2 is composed when the cylindrical portion 8a is calked and expanded in the radial outward direction. The wall thickness of the calking portion 9a is gradually reduced with respect to the wall thickness a0 of the base end portion of the cylindrical portion 8a (FIG. 12) when it comes to the forward end portion. That is, as shown in FIG. 13, the wall thickness of the base end portion of the calking portion 9a is a0, and this wall thickness is changed in the order of a0, a1, a2, . . . an when it comes to the forward end portion. In this case, sections of the protruding portion 16 and the recess portion 17 are regulated to form the calking portion 9a so that the wall thickness of each portion can be kept in the relation of a0 greater than a1 greater than a2 greater than  . . . an and so that the wall thickness an of the forward end portion of the calking portion 9a can not become zero (an greater than 0).
When the die 15 having the protruding portion 16 and the recess portion 17 is incorporated into an oscillating calking device and the die 15 is pressed against the forward end portion of the cylindrical portion 8a, it is possible to calk and expand the forward end portion of the cylindrical portion 8a in the radial outward direction, so that the calking portion 9a can be formed. Therefore, the inner ring 3 can be interposed between the calking portion 9a and the step face 25 of the step portion 7 formed in the inside end portion of the hub body 2, and the inner ring 3 can be fixed to the hub body 2. In the example shown in the drawing, at the final stage of forming the calking portion 9a by plastically deforming the inside end face of the cylindrical portion 8a, a compressive force, the direction of which is in the radial inward direction, is given by the inner face of the recess portion 17 to the outer circumferential face of the calking portion 9a. Accordingly, it is possible to prevent the occurrence of damage such as cracks in the outer circumferential edge of the calking portion 9a. In the inside end opening circumferential edge portion of the inner ring 3 coming into contact with the outer circumferential face of the base end portion of the calking portion 9a, there is provided a chamfer portion 20, the section of which is an arc. Therefore, the radius of curvature of the base end portion of the calking portion 9a is not decreased. Therefore, an excessively high stress is seldom given to the base end portion.
In the conventional method of manufacturing a hub unit for supporting a wheel described in Japanese Unexamined Patent Publication No. 10-272903, it is surely possible to prevent the occurrence of cracks in the outer circumferential edge of the calking portion 9a. However, there is a possibility that cracks are caused on the forward end face or inner circumferential face of the calking portion 9a. In the conventional structure, as shown in FIG. 14, the inclination angle xcex821 of the inner circumferential face 21 of the cylindrical portion 8a is set at a relatively large angle of 20xc2x0, so that a difference between the wall thickness of the base end portion of the cylindrical portion 8a and that of the forward end portion is made large. When this cylindrical portion 8a is plastically deformed by the die 15 which is incorporated into an oscillating calking device, an average tensile stress tends to be generated in the forward end face and the inner circumferential face of the cylindrical portion 8a and the calking portion 9a. When a value of this average tensile stress is increased, cracks are caused in the portion concerned, and the hub body becomes defective, which needs to be discarded. Therefore, the yield is deteriorated or it becomes necessary to repair the portion in which cracks have been caused. In any case, the manufacturing cost is raised.
Further, since the inclination angle xcex821 of the inner circumferential face 21 of the cylindrical portion 8a is large, when this cylindrical portion 8a is pressed by the die 15, a force tends to be given to the base portion of this cylindrical portion 8a in the radial outward direction. When the outer diameter of the cylindrical portion 8a is expanded by this force, the inner ring 3 externally engaged with the step portion 7 existing in the periphery of the cylindrical portion 8a is elastically deformed, and a diameter of the inner ring raceway 11 formed on the outer circumferential face of the inner ring 3 is extended, which could be a cause of a change in the pre-load of the rolling bearing.
The method of manufacturing a hub unit for supporting a wheel and the die for manufacturing it of the present invention have been accomplished to prevent the occurrence of cracks in the calking portion 9a and the extension of the diameter of the inner ring 3.
In the same manner as that of the conventional hub unit for supporting a wheel, the hub unit for supporting a wheel, which is manufactured by the method of manufacturing a hub unit for supporting a wheel of the present invention, includes: a hub body, a first inner ring raceway, a step portion, an inner ring, an outer ring and a plurality of rolling elements.
In the hub body, there is provided a flange, which is formed on an outer circumferential face of one end face, for supporting a wheel.
The first inner ring raceway is formed on an outer circumferential face of a middle portion of the hub body directly or via an inner ring which is formed differently from the hub body.
The step portion is formed in the other end portion of the hub body, and the outer diameter of the step portion is smaller than that of the portion in which the first inner ring raceway is formed.
The inner ring forms a second inner ring raceway on the outer circumferential face and externally engages with the step portion.
On the inner circumferential face of the outer ring, there are provided a first outer ring raceway opposing to the first inner ring raceway and a second outer ring raceway opposing to the second inner ring raceway.
There are respectively provided a plurality of rolling elements between the first inner ring raceway and the first outer ring raceway and also between the second inner ring raceway and the second outer ring raceway.
At the other end portion of the hub body, the calking portion is composed when the cylindrical portion formed in a portion protruding more than the inner ring, which is externally engaged with the step portion, is plastically deformed in the radial outward direction. By this calking portion, the inner ring externally engaged with the step portion is pressed against the step face of the step portion, so that the inner ring externally engaged with the step portion can be connected with and fixed to the hub body.
According to the method of manufacturing a hub unit for supporting a wheel of the present invention, in order to make the above hub unit for supporting a wheel, the cylindrical portion is plastically deformed and formed into the calking portion.
According to the method of manufacturing a hub unit for supporting a wheel of the present invention according to a first aspect of the invention, while the die is being pressed against the forward end face of the cylindrical portion, the die gives forces to the cylindrical portion in the axial direction and the radial outward direction. While this cylindrical portion is being compressed in the axial direction, the forward end portion of the cylindrical portion is plastically deformed in the radial outward direction so that the calking portion can be formed. Therefore, while the calking portion is being formed, an average stress of compression (average compressive stress) is continuously generated on the inner circumferential face of the cylindrical portion.
In this connection, this average compressive stress is hydrostatic stress "sgr"m. This hydrostatic stress "sgr"m can be expressed by "sgr"m=("sgr"1+"sgr"2+"sgr"3)/3, wherein "sgr"1, "sgr"2 and "sgr"3 are longitudinal stress acting in the directions of three axes (axes of x, y and z). When the hydrostatic pressure is p, the expression p=xe2x88x92"sgr"m can be established.
According to the method of manufacturing a hub unit for supporting a wheel according to a second aspect of the invention, while the calking portion is being formed by plastically deforming the cylindrical portion, a portion of metallic material composing the cylindrical portion is moved in the radial inward direction. Therefore, after the calking portion has been formed, an expanding portion, which expands in the radial inward direction, is formed in the inner diameter portion of the calking portion.
On the pressing face of the surface of the die which comes into contact with the cylindrical portion and plastically deforms the cylindrical portion, both of or one of the surface treatment such as ceramic coating of TiN, which enhances the abrasion resistance property, and the treatment such as shot-peening, which reduces surface roughness, is conducted. In this way, the abrasion resistance property is enhanced and the friction coefficient on this pressing face is stabilized at a relatively high value not less than 0.3.
As a die used for executing the method of manufacturing a hub unit for supporting a wheel according to the second aspect of the invention, the die includes: a protruding portion, the shape of which is a circular truncated cone, capable of being freely pushed into the cylindrical portion; and an annular recess portion formed in the periphery of the protruding portion, surrounding the overall circumference of the protruding portion. It is preferable that a shape of a section of the annular recess portion is composed of an inner diameter side arc portion existing in a portion close to the inner diameter and an outer diameter side arc portion existing in a portion close to the outer diameter, the radius of curvature of the outer diameter side arc portion is smaller than that of the inner diameter side arc portion, the inner diameter side arc portion and the outer diameter side arc portion are smoothly continued to each other directly or via a straight line portion. When the annular recess portion is pressed against the forward end face of the cylindrical portion, a center of the radius of curvature of the inner diameter side arc portion does not exist outside the center of the radius of curvature of the outer diameter side arc portion with respect to the radial direction of the cylindrical portion. In other words, with respect to the radial direction of this cylindrical portion, the center of the radius of curvature of the arc on the inner diameter side exists at the same position as the center of the radius of curvature of the arc on the outer diameter side or at a position inside in the radial direction of the center of the radius of curvature of the arc on the outer diameter side. Further, this center of the radius of curvature of the arc on the outer diameter side does not exist outside in the radial direction of the cylindrical portion of the outer circumferential face of the cylindrical portion. That is, with respect to the radial direction of the cylindrical portion, the center of the radius of curvature of the arc on the outer diameter side exists at the same position as the outer circumferential face of the cylindrical portion or inside in the radial direction of the outer circumferential face.
According to the method of manufacturing a hub unit for supporting a wheel of the present invention, a tensile average stress causing cracks is not generated in the cylindrical portion and the inner circumferential portion of the calking portion when the calking portion is machined (the first aspect). Alternatively, an intensity of the tensile average stress causing cracks is reduced even if it is generated (the second aspect). The outer diameter of the base portion of the cylindrical portion is not expanded even when the calking portion is worked. Accordingly, no tensile average stress in the circumferential direction is given to the inner ring externally engaged with the step portion.
Therefore, the durability of the calking portion and the second inner ring raceway can be enhanced, that is, it is possible to provide a hub unit for supporting a wheel, the durability of which is high.
When the calking portion is formed with the die for manufacturing the hub unit for supporting a wheel according to a third aspect, an excellent calking portion can be stably formed.